Cathode structure for electrolytic reduction cell



1953' J. A. MILLER 3,110,660

CATHODE STRUCTURE FOR ELECTROLYTIC REDUCTION CELL Filed Nov. 28, 1960 III A N N INVENTOR. Lo Lo JAMES A. MILLER United States Patent Delaware Filed Nov. 28, 1960, Ser. No. 72,082 4 Claims. (Cl. 204243) This invention relates to improvements in the construction of the cathode structure of an electrolytic reduction cell suitable for the production of aluminum.

Aluminum is commonly produced by a fused salt electrolysis process in which alumina is dissolved in a melt of cryolite. This electrolysis is carried out in a specially constructed reduction cell having an anode structure, a cathode structure, and means by which current can be passed from the anode electrode through the molten contents of the cell to a cathode electrode. In such cells the cathode makes direct contact with the pad of molten aluminum which forms along the bottom of the cell.

The cathode structure is usually composed of several I pants. The cathode shell is a steel container which forms four external sides and an external bottom. Insulation is usually placed on the inside of the shell tolimit heat losses therefrom. Carbon is then lined on the inner side of the insulation to form a cavity which is open on top. This carbon lining is generally referred to as cathode lining or pot lining, while the cavity so formed is termed the cathode cavity. Steel bars are usually embedded in the cathode lining to serve as current collectors and lead the current from the cathode lining to the ex ternal current conductors. Such bars are termed cathode bars.

Thus, it may be seen that the cathode lining serves two functions. Firstly, it must be a current conductor to .carry current from the cathode electrode, which is an aluminum metal pad, to the cathode bars. Secondly, it must be a refractory container for the molten aluminum and molten electrolyte. Since the cathode liner functions as a crucible, it must be liquid tight, and must not develop cracks or holes to allow the melt to leak into or through the carbon container. melt into or through the cathode lining is destructive of the cathode lining, the insulating value of the insulation, the cathode bars and, in extreme cases, the cathode shell.

The cathode lining of an aluminum reduction cell is usually rammed monolithic carbon or large prebaked carbon blocks. The rammed monolithic lining consists of a carbon paste which is rammed into the insulation cavity and around pre-inserted cathode bars, and is then baked by electrical means until the carbon is cured. The pre baked carbons are usually formed by baking green carbon extrusions in special furnaces. The electrical connection from cathode lining is typically accomplished by grooving the carbon in some complicated shape. The cathode bar is inserted in this groove and sealed to the carbon by pouring a molten metal or by ramming a green carbon paste into the groove and around the bar. The fabricating techniques are complicated, and there is still present the probability that the carbon may be weakened by the grooves. Also, it is evident that the carbon thickness must be increased by the depth of the groove in order to provide the desired refractory thickness between the cathode cavity and the cathode collector bars.

It is commonly known in the art that, even though the cathode liner is liquid tight with no large holes or extensive cracks, it has been impossible to make car-bon impervious tothe melt. The electrolyte, after a period of cell operation, will permeate the carbon, and will also migrate into and progressively saturate the insulation. This permeation, migration, and saturation has several effects,

Penetration of the I the major of which is to cause cubical expansion of the carbon and to cause increased thermal conductivity of the insulation. Since the insulating value of the insulation .is thus decreased, it is evident that the thermal stability of the cathode structure is correspondingly decreased. Such a decrease in thermal stability may well result in the deleterious effect of increasing the rate of cubical expansion in the cathode lining.

'It is desirable to achieve a minimum voltage loss in the cathode structure as well as to achieve a maximum operating life of the cathode structure. To overcome these difliculties the prior art has generally proceeded in the direction of larger and more expensive carbon blocks, in some cases with complicated thermal expansion joints, all resulting in expensive and intricate reduction cell cathode structure arrangements.

The present invention provides a simple and relatively inexpensive construction of a cathode structure which eliminates the need for thick carbon blocks having complex shapes or intricate grooves. Also eliminated are complicated electrical connections and the need for thermal expansion joints. On the other hand, a barrier is provided to stop the migration and saturation of electrolyte into the thermal insulation. By using thin, flat-faced carbon slabs arranged in an inverted arch in intimate physical and electrical contact with a metal pan, which functions as a cathode current collector as well as an electrolyte barrier, a longer operating cathode is achieved with better operating stability and lower energy consumption per unit of productivity. The thinness of the carbon cathode liner in conjunction with a metal cathode collector pan apparently results in lower and more uniform electrical resistance and more rapid and uniform distribution of heat, with the consequent avoidance of localized thermal stresses in the cathode liner.

In general, this invention contemplates an electrolytic cell for the production of aluminum in which the cathode structure comprises a cathode shell, thermal insulation, a cathode collector metallic pan, and a carbon cathode liner with a cathode cavity formed therein. The cathode collector pan forms a supporting structure across the entire inner bottom of the cathode structure within which are positioned thin, fiat-faced, pro-baked carbon slabs or blocks. The faces of the blocks across the cell are progressively inclined slightly, and the pan bottom is correspondingly inclined to form an inverted arch, thereby creating additional strength and stability to contain the progressive cubical growth of the cathode lining carbon.

F or better understanding of the invention and its other objects, advantages, and details, reference is now made to the present preferred embodiment of the invention which is shown, for purposes of illustration only, in the accompanying drawings. In the drawings:

FIG. 1 is a sectional elevation view across the narrow dimension of the electrolytic cell.

FIG. 2 is a partial sectional elevation view transverse to the section of FIG. 1 taken along line 22 in FIG. 1.

Referring now to the drawing, an alumina reduction cell 10 is shown which comprises a pot shell 16, supported on any suitable support such as I-beams 12. The bottom of the shell is closed by a metal plate 14, thereby forming a metal casing for the lower portion of the cell. Horizontal beams 20 and 22 reinforce the upper horizontal portion of the shell, and vertical support beams 24 may also be provided. Within the metal casing, insula tion 26 of suitable thickness is provided to thermally insulate the cell. Along the side walls of the cell are shown ram-med carbon side walls 30 and additional thermal insulation 28.

The improvement of this invention relates particularly to the bottom or cathode structure of the cell, comprising a plurality of pre-baked carbon slabs 34 which present a continuous surface with no grooves therein. The carbon slabs 34 are relatively thin, for example as thin as 5 /2 inches although they may be somewhat thicker, and their abutting edges across the narrow dimension of the furnace are inclined about one degree from a perpendicular to the face of each slab so that when assembled in abutting relationship, they form an inverted arch.

The carbon slabs 34 are supported by a metal pan 36 which is made of mild steel, cast iron, ductile cast iron, or cast copper-chrome-iron alloy, for conducting away the current from the carbon. The pan 36 is made up by a number of pan sections 38 each provided with a reinforcing web 40, and downturned edges 42 for attaching each pan to an adjacent similarly shaped pan. The attachment is by means of suitable bolts 44 (with nuts 46) extending through holes in the downturned webs 4-2. Across the short dimension of the furnace, the pan sections are constructed and bolted together in such a manner that they form a generally ilat but slightly depressed bottom of inverted arch shape. That is, the middle of the portion as viewed in FIG. 1 is lower than the sides of the bottom at the edge of the rammed carbon sides 30.

The pan 36 is provided with upturned flanges 48 at the edges thereof as viewed in FIG. 1, for supporting the outer carbon slabs and assisting in forming the downturned arch. The current collecting pan 36 has lateral extensions 50 to which are welded steel collectors 52, attached by suitable conductors 54 to the electrical circuit as is well known in the art of electrolytic reduction cells for producing aluminum.

The interstices between the blocks 34 are filled with a graphite pitch cement '56 or some other suitable electrode cement, and similar cement is used between the bottom of the blocks and the top of the pan 36. This assists in holding the carbon block in place and in providing intimate contact for [good electrical conduction.

Since the thermal coefficient of expansion of the metal from which the pan 36 ismade is somewhat greater than the thermal coefiicient of expansion of the prebaked carbon slabs 34, the pan will not unduly confine the slabs during the initial thermal expansion, which could cause cracking or breaking of the slabs. Also, because the prebaked carbon slabs are relatively thin, i.e., in the nature of less than half as thin as that known in the prior art, there will be substantially no temperature ditierential to retard the expansion of the metal pan relative to the car-- bon lining. Further, because the contact with the current collecting pan 36 is across the entire bottom of all the carbon slabs 34, the voltage drop through the cell is reduced; and since there are no grooves or other intricate connections, there is less danger of the prebaked carbon slabs splitting or cracking. The inverted arch construction of the carbon slabs insures that they will remain in liquid tight contact during the initial thermal expansion without need for any special provision for thermal expansion, while providing a very stable structure to rmist progressive cubical expansion of the carbon lining as'the pot ages due to absorption of electrolyte,

While a present preferred embodiment of the invention has been illustrated and described, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

I claim:

1. An electrolytic cell for the production of aluminum having a chamber adapted to hold the molten contents of the cell, a plurality of prebaked carbon slabs disposed in substantially abutting relationship along the bottom of said chamber, a metallic support pan providing continuous underlying support for the carbon slabs,and a bed of insulation beneath the support pan to maintain said pan and slabs at substantially the same temperature during operation of the cell.

2. An electrolytic cell according to claim 1, comprising a plurality of metallic pan sections detachably joined to form said support pan.

3. An electrolytic cell for the production of aluminum having a chamber adapted to hold the molten contents of the cell, including a pad of molten aluminum; a cathode construction for establishing electrical contact with the aluminum pad, comprising a plurality of flat-faced carbon slabs disposed in substantially abutting relationship along the bottom of said chamber, and a-metallic support pan having a substantially flat but slightly con cave surface providing continuous underlying support for the carbon slabs; and a bed of insulation beneath the support pan to maintain said pan and slabs at substantially the same temperature during operation of the cell.

4. An electrolytic cell for the production of aluminum having a chamber adapted to hold the molten contents of the cell, including a pad of molten aluminum; a cathode construotion for establishing electrical contact with the aluminum pad, comprising a plurality of flat-faced carbon slabs of uniform thickness disposed in substantially abutting relationship along the bottom of said chamber, and a metallic support pan having a substantially fiat but slightly concave surface providing continuous underlying support for the carbon slabs,said support pan having upturned flanges extending along opposite sides of the pan to provide lateral confinement of the slabs and resist progressive cubical expansion of the carbon; and a bed of insulation beneath the support p an to maintain said pan and slabs at substantially the same temperature during operation of the cell.

References Cited in the file of this patent UNITED STATES PATENTS 673,364 Hoopes Apr. 30, 1901 938,634 Betts Nov. 2, 1909 1,369,578 Trembour Feb. 122, 1921 2,593,751 Grolee Apr.'22, 1952 2,861,036 Simon-Suisse Nov. 18, 1958 2,980,596 Conway Apr. 18, 1961 FOREIGN PATENTS 1,059,192 Germany June 11, 1959 

4. AN ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM HAVING A CHAMBER ADAPTED TO HOLD THE MOLTEN CONTENTS OF THE CELL, INCLUDING A PAD OF MOLTEN ALUMINUM; A CATHODE CONSTRUCTION FOR ESTABLISHING ELECTRICAL CONTACT WITH THE ALUMINUM PAD, COMPRISING A PLURALITY OF FLAT-FACED CARBON SLABS OF UNIFORM THICKNESS DISPOSED IN SUBSTANTIALLY ABUTTING RELATIONSHIP ALONG THE BOTTOM OF SAID CHAMBER, AND A METALLIC SUPPORT PAN HAVING A SUBSTANTIALLY FLAT BUT SLIGHTLY CONCAVE SURFACE PROVIDING CONTINUOUS UNDERLYING SUPPORT FOR THE CARBON SLABS, SAID SUPPORT PAN HAVING UPTURNED FLANGES EXTENDING ALONG OPPOSITE SIDES OF THE PAN TO PROVIDE LATERAL CONFINEMENT OF THE SLABS AND RESIST PROGRESSIVE CUBICAL EXPANSION OF THE CARBON; AND A BED OF INSULATION BENEATH THE SUPPORT PAN TO MAINTAIN SAID PAN AND SLABS AT SUBSTANTIALLY THE SAME TEMPERATURE DURING OPERATION OF THE CELL. 